A fuel cell is a device for generating electricity directly from the electrochemical reaction of oxygen with either hydrogen or the hydrogen found in hydrocarbon materials such as methanol, ethanol, or natural gas.
A fuel cell can be classified into one of the following types: a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type depending upon the kind of electrolyte used. Although each fuel cell basically operates in accordance with the same basic principles, the type of fuel cell may determine the kind of fuel, the operating temperature, the catalyst, and the electrolyte that are used.
Recently, polymer electrolyte membrane fuel cells (PEMFC) have been developed that have superior power characteristics, lower operating temperatures, and faster start and response characteristics compared to conventional fuel cells. They have advantages since they can be applied to a wide range of fields, such as transportable electric sources for automobiles, distributed power sources, for example for houses and public buildings, and small electric sources for electronic devices.
The PEMFC is essentially composed of an electricity generator, a reformer, a fuel tank, and a fuel pump. The fuel pump provides fuel stored in the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas and supplies the hydrogen gas to the electricity generator where it is electrochemically reacted with oxygen to generate electrical energy.
Another type of fuel cell is a direct oxidation fuel cell (DOFC) such as a direct methanol fuel cell (DMFC) in which liquid methanol fuel is directly introduced to the electricity generator. The DMFC can omit the reformer, which is essential for a polymer electrolyte fuel cell.
According to the fuel cell system described above, the electricity generator in a fuel cell system generates electricity and has a layered structure (referred to as stack) consisting of from several to tens of unit cells. Each unit cell is composed of a membrane-electrode assembly (MEA) and two separators (or bipolar plates). The MEA has a structure with a polymer electrolyte membrane interposed between the anode (referred to as fuel electrode or oxidation electrode) and the cathode (referred to as air electrode or reduction electrode).
The separators not only work as passageways for supplying the fuel required for the reaction to the anode and for supplying oxygen to the cathode, but also as conductors serially connecting the anode and the cathode in the MEA. An electrochemical oxidation reaction of the fuel occurs at the anode, and an electrochemical reduction reaction of oxygen occurs at the cathode, thereby producing electricity, heat, and water, due to the migration of electrons generated during this process.
Generally, the anode and cathode of a fuel cell contain a platinum (Pt) catalyst. However, since platinum is an expensive noble metal, it is expensive to use in large quantities. Therefore, platinum supported by carbon has been used to reduce the amount of platinum used.
However, carbon-supported platinum catalyst makes the catalyst layer thick and has a limited storage capacity. Moreover, poor contact between the catalyst layer and the electrolyte membrane degrades the performance of the fuel cell.
Also, using a carbon-supported platinum catalyst may cause problems in that part of the carbon-supported platinum catalyst is buried in a binder resin during the electrode fabricating process, and such buried catalyst is incapable of participating in the desired catalytic reactions.
Therefore, there are benefits in developing an electrode for a fuel cell that uses a reduced amount of catalyst in the electrode while still showing excellent cell performance.